Casing joints, liners, and other oilfield tubulars are often used in drilling, completing, and producing a well. Casing joints, for example, may be placed in a wellbore to stabilize a formation and protect a formation against high wellbore pressures (e.g., wellbore pressures that exceed a formation pressure) that could damage the formation. Casing joints are sections of steel pipe, which may be coupled in an end-to-end manner by threaded connections, welded connections, and other connections known in the art. The connections are usually designed so that a seal is formed between an interior of the coupled casing joints and an annular space formed between exterior walls of the casing joints and walls of the wellbore. The seal may be, for example, an elastomer seal (e.g., an o-ring seal), a thread seal, a metal-to-metal seal formed proximate the connection, or similar seals known in the art.
One type of threaded connection commonly used to form a thread seal in oilfield tubulars is a wedge thread. In FIGS. 1A and 1B, a prior art connection having a wedge thread is shown. “Wedge threads” are characterized by threads, regardless of a particular thread form, that increase in width in opposite directions on a pin member 101 and a box member 102. The rate at which the threads change in width along the connection is defined by a variable commonly known as a “wedge ratio.” As used herein, “wedge ratio,” although technically not a ratio, refers to the difference between the stab flank lead and the load flank lead, which causes the threads to vary in width along the connection. A detailed discussion of wedge ratios is provided in U.S. Pat. No. 6,206,436 issued to Mallis, and assigned to the assignee of the present invention. That patent is incorporated herein by reference in its entirety.
Wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647 issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No. 4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention and incorporated herein by reference. Continuing with FIGS. 1A and 1B, on the pin member 101, a pin thread crest 222 is narrow towards the distal end of the pin member 101 while a box thread crest 291 is wide. Moving along an axis 105 (from right to left), the pin thread crest 222 widens while the box thread crest 291 narrows. Referring still to FIGS. 1A and 1B, the threads are tapered, meaning that a pin thread 106 increases in diameter from beginning to end while a box thread 107 decreases in diameter in a complimentary manner. Having a thread taper can improve the ability to stab the pin member 101 into the box member 102 and distributes stress in the connection.
Generally, thread seals are difficult to achieve with non-wedge threads having broad crests and roots, however, the same thread forms may have thread seals when used for wedge threads. Wedge threads do not have any particular thread form. One example of a suitable thread form is a semi-dovetailed thread form disclosed in U.S. Pat. No. 5,360,239 issued to Klementich, and incorporated herein by reference. Another thread form includes a multi-faceted load flank or stab flank, as disclosed in U.S. Pat. No. 6,722,706 issued to Church, and incorporated herein by reference. Each of the above thread forms is considered to be a “trapped” thread form, meaning that at least a portion of the corresponding load flanks and/or corresponding stab flanks axially overlap. An open (i.e. not trapped) thread form with a generally rectangular shape is disclosed in U.S. Pat. No. 6,578,880 issued to Watts. The above thread forms are examples of thread forms that may be used for embodiments of the invention. Generally, open thread forms such as buttress or stub are not suitable for wedge threads because they would impart a large radial force on the box member. A generally square thread form, such as that disclosed by Watts, or a trapped thread form does not impart an outward radial force on the box member. Those having ordinary skill in the art will appreciate that the teachings contained herein are not limited to particular thread forms.
For wedge threads, a thread seal may be accomplished as a result of the contact pressure caused by interference over at least a portion of the connection between the pin load flank 226 and the box toad flank 225 and between the pin stab flank 232 and the box stab flank 231, which occurs when the connection is made-up. Close proximity or interference between the roots 292 and 221 and crests 222 and 291 completes the thread seal when it occurs over at least a portion of where the flank interference occurs. Generally, higher pressure may be contained wit increased interference between the roots and crests (“root/crest interference”) on the pin member 101 and the box member 102 and by increasing flank interference. The particular connection shown in FIG. 1A also includes a metal-to-metal seal that is accomplished by contact pressure between corresponding seal surfaces 103 and 104, respectively located on the pin member 101 and box member 102.
Wedge threads typically do not have a positive stop torque shoulder on the connection. For wedge threads that do not have a positive stop torque shoulder, the make-up is “indeterminate,” and, as a result, the relative position of the pin member and box member varies more during make-up for a given torque range to be applied than for connections having a positive stop torque shoulder. As used herein, “make-up” refers to threading a pin member and a box member together. “Selected make-up” refers to threading the pin member and the box member together with a desired amount of torque, or based on a relative position (axial or circumferential) of the pin member with the box member. For wedge threads that are designed to have both flank interference and root/crest interference at a selected make-up, both the flank interference and root/crest interference increase as the connection is made-up (i.e. increase in torque increases flank interference and root/crest interference). For tapered wedge threads that are designed to have root/crest clearance, the clearance decreases as the connection is made-up. Regardless of the design of the wedge thread, corresponding flanks come closer to each other (i.e. clearance decreases or interference increases) during make-up. Indeterminate make-up allows for the flank interference and root/crest interference to be increased by increasing the make-up torque on the connection. Thus, a wedge thread may be able to thread-seal higher pressures of gas and/or liquid by designing the connection to have more flank interference and/or root/crest interference or by increasing the make-up torque on the connection, however, this also increases stress on the connection during make-up, which could lead to failure during use.
Free-running threads used for oilfield tubular connections typically do not form thread seals when the connection is made-up. FIG. 2 shows a prior art connection having free-running threads. The free-running threads include load flanks 154 and 155, stab flanks 157 and 158, crests 159 and 162, and roots 160 and 161. As is typical of a connection with free-running threads, this connection relies on a positive stop torque shoulder formed by the contact of surfaces 151 and 152 disposed on the pin member 101 and the box member 102, respectively. The positive stop torque shoulder shown in FIG. 2 is commonly referred to as a “pin nose shoulder.” In other connections, the positive stop torque shoulder may instead be formed by the box face 163 and a mating shoulder (not shown) on the pin member 101. The positive stop torque shoulder also provides a seal. Unlike wedge threads, which make-up by the wedging of the pin thread 106 and the box thread 107, free-running threads rely on the positive stop torque shoulder to load the connection during make-up. To make-up the connection shown in FIG. 2, the pin member 101 and the box member 102 are screwed together until the surfaces 151 and 152 are brought into abutment, at which point the pin load flank 154 and box load flank 155 are also in abutment. Additional torque is applied to the pin member 101 and the box member 102 to load the surfaces 151 and 152 and the pin load flank 154 and box load flank 155 until the desired amount of make-up torque has been applied to the connection.
The connection shown in FIG. 2 does not accomplish a thread seal because of the large gap 153 that exists between the pin stab flank 157 and box stab flank 158. The gap 153 occurs because of how free-running threads with positive stop torque shoulders are loaded. Applying torque to the connection during make-up against the positive stop torque shoulder causes the pin member 101 to be compressed while the box member 102 is stretched in tension. Note that when a box face shoulder is used, the box member 102 is compressed while the pin member 101 is stretched in tension. The force between the pin member 101 and the box member 102 is applied through the pin load flank 154 and box load flank 155. The pin stab flank 157 and the box stab flank 158 are not loaded during make-up. This results in contact pressure between the load flanks 154 and 155 and a gap between stab flanks 157 and 158. As discussed above, a wedge thread (as shown in FIGS. 1A and 1B) is able to form a thread seal in part because of the interference between the load flanks 225 and 226 and the stab flanks 232 and 231. For wedge threads, this occurs near the end of the make-up of the connection because of the varying width of the pin thread 106 and the box thread 107. To have similar interference between the load flanks 154 and 155 and stab flanks 157 and 158 on a cylindrical (i.e. non-tapered) free-running thread and the thread heights are continuous, the interference would exist substantially throughout the make-up of the connection because the pin thread 106 and the box thread 107 have a continuous width. Because the particular connection shown in FIG. 2 is cylindrical (i.e. not tapered), root/crest interference, if any, would exist substantially throughout the make-up of the connection. This could lead to galling of the threads and difficulty in making up the connection.
In the prior art, the angles at which the load flank and the stab flank are disposed relative to the central axis of the threaded connection are constant. As a result, a thread either has an open or trapped thread form for its entire length. Each thread form has advantages depending on its application. For example, a buttress thread form can be advantageous for a shouldered non-wedge connection that will experience a large amount of axial tension and relatively little axial compression. For wedge threads, a dovetailed thread form is often used because it pulls the pin member and the box member together during make-up. Also, an open thread form with positive flank angles can impart large radial forces as a result of the wedging force between the pin thread and the box thread.
In some connections, more than one thread form may be advantageous; however, one thread form must be selected over the other. The advantage of multiple thread forms in a connection was recognized in U.S. Pat. No. 6,767,035 issued to Hashem. The '035 patent discloses a shouldered connection having multiple threads formed on both the pin member and the box member. The cutting of multiple threads, sometimes referred to as a “multi-start” thread, results in a threaded connection that has an alternating open and closed thread form in a two-dimensional cross-section. Embodiments disclosed in the '035 patent have two threads formed on the pin member and the box member. One thread has an open thread form, such as a stub acme. The other thread has a trapped thread form, such as dovetailed or hooked. Such a combination of thread forms results in advantages of both thread forms existing uniformly over the length of the connection.
The stresses experienced by a threaded connection are not uniform over the length of a thread, for example, during bending of the distal ends and the middle portion of the threaded connection. As a result, a particular thread form can be the best for one part of the connection, but not for another. Thus, what is still needed is a threaded connection that allows for the thread form to be selected over different portions of the connection to distribute stresses.